syzbot suspects this bug was fixed by commit:

commit 32c72165dbd0e246e69d16a3ad348a4851afd415
Author: Kadlecsik József [off-list ref]
Date:   Sun Jan 19 21:06:49 2020 +0000

    netfilter: ipset: use bitmap infrastructure completely

bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=16b8e545e00000
start commit:   d96d875e Merge tag 'fixes_for_v5.5-rc8' of git://git.kerne..
git tree:       upstream
kernel config:  https://syzkaller.appspot.com/x/.config?x=cf8e288883e40aba
dashboard link: https://syzkaller.appspot.com/bug?extid=6491ea8f6dddbf04930e
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=126748d6e00000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1385f959e00000

If the result looks correct, please mark the bug fixed by replying with:

#syz fix: netfilter: ipset: use bitmap infrastructure completely

For information about bisection process see: https://goo.gl/tpsmEJ#bisection